Cathode



Jan. 28, 1964 D. R. KERSTETTER ETAL CATHODE Filed May 5, 1961 INVENTOR5.Dana/a R Kerseher Samue/JManfgamery ATTORNEY United States Patent3,119,946 CATHODE Donald R. Kerstetter and Samuel .1. Montgomery, Em-

porium, Pa., assignors to Sylvania Electric Products Inc., a corporationof Delaware Filed May 5, 1961, Ser. No. 108,118 4- Claims. (CL 313337)This invention relates to indirectly heated cathodes employed inelectron tubes.

In such cathodes there is often used a metallic sleeve exteriorly coatedwith an electron emissive material and, upon occasion, interiorly coatedwith an insulating medium such as aluminum oxide. Within this coatedsleeve there is positioned an insulatingly coated heater.

Very often, due to the thinness or absence of insulating coating on theinterior of the sleeve and the poor condition of the heater insulation,the heater wire is improperly electrically isolated from the metalsleeve and leakage between the heater and metal sleeve occurs. This isparticularly evident and undesirable when the electron tube is used inpulsed applications, in which use, in some circuits, high potentials mayexist between the heater and cathode.

In an attempt to overcome this condition, thicker insulation between acoated heater coil and sleeve has been proposed and even a secondinsulated wire coil has been Wound about the insulated heater coil andbonded thereto for insertion into the sleeve. Both of theseconstructions are expensive to fabricate and they do not lend themselvesto consistent reproducibility from tube to tube, of the desired heattransfer, microphonism and heather-cathode leakage characteristics.

Accordingly, it is an object of this invention to inexpensively achieveexcellent insulation and reproduceable heat transfer characteristicsbetween the heater wire and the cathode sleeve in a heater-cathodeassembly.

In carrying out the invention, a length of wire, either bare orinsulated, subsequently to become an insulating structure, is coiledabout a mandrel after which the resilient spacer coil is loosened byallowing it to partially uncoil and expand to enable the mandrel to beremoved. An insulated, folded, or coiled heater is subsequently insertedwithin the void left by removal of the mandrel. The assembly of innerheater and outer spacer coil is then inserted into a cathode sleeve.After this insertion, outer portions of the turns of the spiral come incontact essentially at points only with the inner wall of the sleeve,while inner portions of the turns engage and hold the heater in place.The resulting structure is a heatercathode assembly with predictablepoint area contact of the insulating coil and the cathode sleeve andpredictable bundling and spacing of the heater from the sleeve. Thesefeatures provide consistent relative positioning of the heater andcathode assembly components and minimization of leakage between theheater and cathode sleeve.

For a better understanding of the invention, attention is directed tothe following detailed specification when taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a perspective view of a mandrel about which is wound a lengthof spacer coil wire;

FIG. 2 is an exploded view of the cathode heater assembly showing thespacer coil, a heater ready for insertion into the coil and acylindrical cathode sleeve in position to receive these elements;

FIG. 3 is an end view of the complete cathode-heater assembly; and

FIG. 4 is a perspective view of the cathode-heater assembly.

Referring to the drawings with greater particularity,

FIG. 1 shows a mandrel 10 having a square cross-section although otherpolygonal shaped mandrels having relatively sharp angular edges may beemployed. As will be hereinafter pointed out, mandrels having anelliptical cross section or a round mandrel having a series of diametersof repetitive variance may also be used. About the mandrel 10 in FIG. 1is wound an insulated wire 12 fabricated from refractory metals such astungsten, molybdenum or alloys thereof. The insulating coil is hereshown as using a resilient wire 14 coated with a suitable insulatingcoating 16 such as aluminum oxide.

The coated wire 12 is helically wound about the angular mandrel 10 withtension not exceeding the elastic limit of the wire since a degree ofresiliency is desired. To facilitate winding, the free end of the wiremay be temporarily secured to the mandrel 10 by suitable clips 18. Thetight winding of insulated wire 12 about the mandrel forms relativelysharp corners on the helix at the corners of the mandrel 10, whichcorners are desirable in the final cathode assembly for positioning theheater 21 relative to the cathode sleeve 22. The length of wire woundmay encompass the requirements of several individual coils 12, thefinished coil being cut to given lengths approximating the length of thecathode cylinder 22. Upon removal from the mandrel 10, the spiral willrelax and form a structure with angular bends or peripheral portions 13interconnected by substantially straight-sided inner portions 15. Thisoutside dimension defined by the bends preferably exceeds the insidediameter of the cathode sleeve 22.

If so desired, Wire 14 may be wound as bare wire, and upon removal fromthe mandrel It may then be coated with the insulating material 16 by anyconventional process such as dipping or electrophoresis.

During assembly, insulated heater 21, illustrated as being of the spadewound type, is bunched and inserted into the central opening or void inthe spiral of the insulating coil 12. If desired, a coating of alundumor other insulating material may be applied to these assembled elementsby dip or electrophoretic processes to form an integral sub-assembly.

The sub-assembly of integrated components is then inserted into thecathode sleeve 22 by bundling or carefully depressing the spiral 12, andthreading, preferably by circular motion compatible with the helixdirection of the heater spacer coil, into the cathode sleeve 22. Sincethe relaxed diameter of the spiral 12 exceeds the inside diameter of thesleeve 22, the spiral, after insertion, will press against the interiorsleeve surface and frictionally hold the heater in appropriate spacedrelationship in the central area of the coil, as can be seen in FIGS. 3and 4.

To achieve a greater degree of insulation, the interior surface of thecathode sleeve may be covered with an insulating coating of aluminumoxide 24 or the like. Thus, the cathode assembly formed, as shown inFIGS. 3 and 4, provides a frictionally mounted resilient spacer betweenthe heater 21 and the cathode sleeve 22 wherein the angular spacer coilhas only point random contact with the sleeve 22 or its insulatingcoating 24 at the bends or peripheral portions 13 while the straightinner turn portions 15 of the coil hold the heater centrally of thesleeve substantially along its longitudinal axis, and removed from thewall of the sleeve.

If desired an elliptically wound coil could be formed so that whenrelaxed the smaller are portions would make random peripheral contactwith the inner cathode wall while the larger arc portions of the ellipsewould contact and support the heater centrally of the sleeve. Also, amodification of the coil spacer could be used which comprises aconvoluted insulating structure of repetitive hour-glass contour. Such acoil could be produced on the above described round mandrel which has aseries of diameters of repetitive variance. The coil would, upon partialrelaxation, aiford spacing and support within the cathode. The largerdiameter outer spirals would bear against the inner cathode wall Whilethe smaller internal spirals would centrally support the heater.

The exterior of the cathode sleeve 22 may be coated in any desiredfashion with an electron emissive coating material 26.

The cathode assembly described herein has a firmly bundled heater 21spaced substantially centrally of sleeve 22. Such a structure providespredictable uniform heat transfer characteristics, and minimizesmicrophonism and heater-cathode leakage. The structure is relativelyinexpensive and lends itself to simple fabrication techniques.

Although several embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thescope of the invention as defined by the appended claims.

What is claimed is:

1. A cathode assembly comprising a substantially cylindrical cathodesleeve provided with an exterior electron emissive coating, a heatermounted within said sleeve, and a resilient spacer coil positionedbetween the heater and said sleeve, the coil comprising an insulatedwound Wire spiral having randomly positioned exterior portions defininga peripheral surface and randomly positioned internal portions definingan interior surface, said peripheral portions bearing against the innerwall of the cathode sleeve and said internal portions engaging theheater to provide confinement thereof and spacing from the sleeve.

2. A cathode assembly comprising a cylindrical cathode sleeve coated onits exterior with an electron emissive material, a heater positionedwithin the sleeve, and a resilient spacer coil lying between the heaterand sleeve, said coil comprising an insulated wound wire with angularbends interconnected by substantially straight-sided portions, theangular bends bearing against the inner wall of the sleeve at randompoints therearound and the heater lying and held within saidstraight-sided portions of the coil at a position spaced from the sleeveand extending substantially along the longitudinal axis thereof.

3. A cathode assembly comprising a cylindrical metallic cathode sleevecoated with electron emissive material on its exterior wall and withinsulation material on its interior wall, a heater, a resilientinsulater heater spacer coil lying inside the sleeve, said coil havingstraight sides interconnected by sharp bends, said bends engaging theinsulated inner wall of the sleeve at random points and the straightwalls of said coil engaging and holding the heater between them at aposition spaced from the sleeve and extending substantially along thelongitudinal axis thereof.

4. A cathode assembly comprising a hollow metallic sleeve with an innerwall devoid of sharp bends, a bunched heater positioned substantiallycentrally within the sleeve and extending substantially along thelongitudinal axis of the sleeve, and a resilient spacer frictionallymounted between the sleeve and heater comprising a metallic resilientwire enclosed in an electrical insulating material, said spacer beinggenerally a spiral with straight-sided portions joined by sharp bends,said bends and straight portions being randomly located about thelongitudinal axis of the sleeve, the sharp bends of the spiral lying incontact with the inner wall of the sleeve, and the heater being engagedand held by said straight-sided portions of the spacer.

References Cited in the file of this patent UNITED STATES PATENTS2,831,140 Blickwedel et al Apr. 18, 1958

1. A CATHODE ASSEMBLY COMPRISING A SUBSTANTIALLY CYLINDRICAL CATHODESLEEVE PROVIDED WITH AN EXTERIOR ELECTRON EMISSIVE COATING, A HEATERMOUNTED WITHIN SAID SLEEVE, AND A RESILIENT SPACER COIL POSITIONEDBETWEEN THE HEATER AND SAID SLEEVE, THE COIL COMPRISING AN INSULATEDWOUND WIRE SPIRAL HAVING RANDOMLY POSITIONED EXTERIOR PORTIONS DEFININGA PERIPHERAL SURFACE AND RANDOMLY POSITIONED INTERNAL PORTIONS DEFININGAN INTERIOR SURFACE, SAID PERIPHERAL PORTIONS BEARING AGAINST THE INNERWALL OF THE CATHODE SLEEVE AND SAID INTERNAL PORTIONS ENGAGING THEHEATER TO PROVIDE CONFINEMENT THEREOF AND SPACING FROM THE SLEEVE.